The objectives of this research are to understand the mechanisms by which pyrimidines are alkylated and halopyrimidines dehalogenated. Several enzyme systems important in cancer and cancer chemotherapy are involved in pyrimidine alkylation reactions. These include thymidylate synthetase, dCMP hydroxymethylase, and the RNA (DNA) methylases. Considerable evidence indicates that these reactions proceed by a mechanism that involves formation of a convalent enzyme- dihydropyrimidine intermediate. Experiments are described, which should demonstrate the existence of such an intermediate in both thymus gland thymidylate synthetase and dCMP hydroxymethylase. These experiments include purification by affinity chromatography, kinetic analysis, and possible isolation of this intermediate. Derivatives of 5-I- and 5-Br-uracil are important antiviral agents which, upon dehalogenation, lose therapeutic effectiveness. The pathway for I-Ura dehalogenation appears to involve TPNH-linked dihydrouracil dehydrogenase reduction to yield 5-I-5,6-dihydrouracil which is subsequently dehalogenated by a newly-discovered enzyme, 5-I-5,6- dihydrouracil dehalogenase. Experiments are described which will result in the isolation and characterization of this enzyme and the reaction catalyzed. A variety of mammalian cell lines, some of which can be induced by I-UdR to produce C-type viruses, will be screened for pyrimidine dehalogenating activity. Chemical model systems will continue to be kinetically studied in relation to enzymatic catalysis. These reactions have environmental impact since nucleophiles, such as bisulfite, are mutagenic, are used in the food industry, and can result from the hydration of SO2, a common air pollutant.